If the Fastpass line is the
only accessible entrance you are allowed to use it with a wheelchair or ecv without having a Fastpass at WDW. A few attractions are like this, but most were either built with accessible lines or had it added. All the Fastpass lines are accessible.
For attractions that have Mainstream lines (which means that the regular line is accessible to wheelchairs and
ECVs), people using wheelchairs or ecvs are able to wait in the regular line and are expected to get a Fastpass if they want to avoid waiting in line. If you want to enter the Fastpass line for Mainstream attractions, you need to have a Fastpass or a specific GAC (most GACs don't allow this because most people don't need that level of access).
Someone who only needs an accessible line, doesn't need a GAC. The CMs can see the wheelchair or ecv and know that you need the accessible line (which may be the regular line).
If there are other needs
besides having an accessible line, that's what a GAC is for. It's a
tool to let CMs know what assistance/accomidation is needed.
Not all GACs are alike. Some have messages that allow a wait in a quieter place, some allow a stroller to be used as a wheelchair, some allow a wait out of the sun (when the line is in the sun for a "prolonged period" - according to the card), some allow use of alternate entrances. The GAC card has some basic information printed on it, then a stamp is added for the guest. What is stamped on the card is based on the needs that the guest explained to Guest Services.
In most cases, the GAC doesn't allow use of the Fastpass return line (although in some cases, that is the alternative entrance). Here's what actually is printed right on the back of all GACs:
Some attractions have alternate entrances for Guests with disabilities. These are intended to offer Guests in wheelchairs or with service animals a more convenient entrance to the attraction. Alternate entrances are not intended to provide immediate access. Guests with disabilities and up to 5 members of their party may enter through these entrances. For operational considerations, additional party members are required to use the standard queue.
At attractions offering Disney's FASTPASS service, Guests should obtain a Disney's FASTPASS return ticket.
Where it becomes confusing is if a Guest goes first to some of the attractions that don't have Mainstream access. MK is the oldest park and has the most attractions with alternate entrances. A lot of people go to MK first and their first experiences are with the least accessible attractions. If they are routed to the exit or Fastpass line for those attractions, they assume that's how it's supposed to work for all attractions. So, when they go to an attraction that
does have Mainstream access and they are directed to either get a Fastpass or use the standard line, it seems like the CM is doing something wrong. So, people assume that they need a GAC to use the wheelchair/ecv in line.
There is a list of which attractions have Mainstream Access on the
official Disney website links to their Guidebook for Guests with Disabilities. The list is way at the bottom of the page.
For those attractions, wheelchair/ecv users are able to use the regular line.
If you are entering a line that does
not have Mainstream access and the CM is refusing to let you use the accessible entrance, then you should ask to speak to a supervisor. But, in general, if it's a Mainstream line and they won't let you go into the Fastpass line without a Fastpass or a specific GAC, they are doing what they are supposed to do.